In the US, 50 percent of patients undergo "curative" resection for colorectal cancer develop recurrent disease. In part, this reflects the inability to detect micrometastatic tumor cells in lymph nodes at the time of staging. Precise staging is crucial because it determines prognosis and influences management including administration of adjuvant chemotherapy. Guanylyl cyclase C (GCC) is specifically expressed only by normal intestinal mucosal and colorectal cancer cells. Indeed, GCC expression, measured by RT-PCR (GCC RT-PCR), was detected in primary and metastatic colorectal tumors, but not in any extra-intestinal tissues and tumors, suggesting that GCC may have utility as a specific biomarker for colorectal cancer staging. GCC RT-PCR detected micrometastases in fresh lymph nodes from Dukes B colorectal cancer patients that were free of tumor by histopathology. In addition, a retrospective case-control study revealed that for all patients with Dukes B (lymph node-negative) colorectal cancer, free of recurrence for >5 years, lymph nodes examined by GCC RT-PCR also were negative. In contrast, for all patients with stage B disease who developed recurrences in <3 years, lymph nodes from the original resection were positive by GCC RT-PCR analysis. The odds ratio for mortality associated with GCC RT-PCR (+) regional lymph nodes was 16.5 (95 percent CI, 1.1-756.7). Thus, GCC RT-PCR appears to be a sensitive and specific method for detecting clinically significant colorectal micrometastases in lymph nodes. GCC RT-PCR may improve the staging accuracy, establishing prognoses, and selecting patients to receive adjuvant chemotherapy. The objective of this proposal is to define the clinical utility of GCC for staging patients with colorectal cancer in a prospective clinical trial. In the first Specific Aim, lymph nodes from about 2,000 patients with colorectal cancer will be examined by histopathology and GCC RT-PCR and their comparative accuracy for micrometastasis detection assessed. It is anticipated that GCC RT-PCR will identify more lymph nodes as "positive," presumably reflecting the presence of micrometastases below the detection limit of conventional analysis. Quantitative GCC RT-PCR will define the burden of metastatic tumor cells in extra-intestinal sites. In Specific Aim 2, the prognostic value of staging by GCC RT-PCR will be compared to standard histopathology. Patients will be followed longitudinally to correlate stage, defined by the 2 techniques, with the development of recurrent disease. Specifically, these studies will define the prognostic value of staging by GCC RT-PCR or histopathology and the relationship between tumor burden, qualitatively defined as the number of lymph nodes containing colorectal cancer cells or quantitatively defined by the number of cancer cells in lymph nodes, and prognosis. In the third Specific Aim, a model will be constructed which incorporates the results of GCC-based tissue assays, along with other accepted prognostic feature, to predict the likelihood of colorectal cancer recurrence. These studies will define the characteristics of GCC RT-PCR as a marker for staging patients with colorectal cancer and will form the basis of future prospective clinical trials to examine the utility of this marker for identifying patients suitable for adjuvant therapies.